This invention relates generally to power supply circuits, and in particular to an automatic power turn-on circuit for a battery-powered voltage measurement apparatus.
Conventional battery-powered voltage measuring instruments, such as voltmeters and the like, typically have power switches that connect and disconnect internal batteries to and from measurement processing circuits in order for them to operate and display the measurement. Typically associated with this type of instrument is a pair of test leads, one for providing a common reference such as ground, and the other for receiving the voltage to be measured. An operator must first turn the power on and then touch the test leads to desired points in the electrical circuit being measured. Of course, all the time that the instrument is turned on, the batteries are draining. If the instrument is left on for an extended period of time without making measurements, or if the instrument is unintentionally left on, battery life is shortened appreciably.
Continuity checkers are known in the art that are powered by a circuit under test and do no more than energize an indicator light. Another prior art approach is that of the type that is taught in U.S. Pat. No. 3,919,631, which discloses a battery-powered leadless probe that detects the presence of voltage by means of a sensor or conductive tip connected to an electronic switch that connects a battery to a light bulb. These prior art devices merely indicate that a voltage is present, and turn off as soon as they are removed from the voltage field.
It would be desirable to provide a battery-powered apparatus capable of automatically turning on in order to provide measurement or indication of a number of different voltages over a predetermined voltage range. Moreover, it would be desirable to provide a battery-powered apparatus that would remain turned on for a predetermined time period following a voltage measurement so that a series of measurements could be made without the apparatus turning on and off for each measurement.